The former promoter of unified junior welterweight world champion Terence Crawford has filed suit against promoter Top Rank, alleging it has refused to pay him what it owes him on each of Crawford's past four title defenses -- roughly $550,000.

Middendorf Sports, owned by Maryland promoter Chris Middendorf, filed a lawsuit on Sept. 1 in U.S. federal court for the district of Nebraska, Crawford's home state. The suit, a copy of which was obtained by ESPN, also names Crawford as a defendant but said he "is named as a nominal defendant as Middendorf Sports does not allege any wrongdoing by Crawford."

What Middendorf does allege is that Top Rank owes him a fee equal to 8 percent of the purses that Crawford was paid for his past four title defenses, and that those fees, including portions of the gate receipts, were due within five business days of each bout but have not been paid.

"Prior to July 23, 2016, Top Rank paid Middendorf Sports without fail" under their deal, the suit said, including money from Crawford's lightweight title defenses against Yuriorkis Gamboa and Raymundo Beltran in 2014 and junior welterweight title defenses against Dierry Jean in 2015 and Hank Lundy in 2016.

But for the next four junior welterweight title defenses that followed the suit alleges that "without legal justification, Top Rank has failed and refused to pay Middendorf Sports the Title Defense Fee" that was agreed to.

The suit said Top Rank owes Middendorf Sports a title defense fee on four of Crawford fights:

A July 2016 title unification fight against Viktor Postol ($1.3 million purse);

A December 2016 defense against John Molina ($1.5 million purse plus gate money);

A May defense against Felix Diaz ($1.6 million purse plus gate money);

An Aug. 19 unification fight against Julius Indongo ($2 million plus gate money) in which Crawford became the 140-pound division's undisputed champion and only the third fighter of the four-belt era to hold all of them simultaneously until vacating one of them 11 days later.

Middendorf became involved with Crawford (32-0, 23 KOs), now universally considered one of the best fighters in the world pound-for-pound, when Middendorf worked for TKO Boxing Promotions, the fledgling company that signed Crawford in 2010.

In 2011, with TKO Boxing Promotions failing, the company negotiated with Top Rank to take over as Crawford's promoter and on June 16, 2011, "with TKO's blessing and approval, Top Rank and Crawford entered into a Promotional Rights Agreement," according to the suit.

Two weeks later, according to the suit, Top Rank's in-house attorney drafted an agreement between TKO and Top Rank under which TKO would be paid the 8 percent fee based on Crawford's purses for any title defenses going forward. On July 19, 2011, according to the suit, TKO assigned its rights to Middendorf Sports under a contract drafted by Top Rank.

"This Assignment was drafted for the benefit of TKO and Middendorf Sports by Top Rank's in-house counsel," the suit said.

According to the suit, Top Rank and Crawford extended their promotional agreement for a minimum of three years on Sept. 16, 2014.

"I don't comment on cases that are currently underway," attorney Don Campbell, who is representing Top Rank, told ESPN on Tuesday. "I'm a former justice department prosecutor and I learned at a very young age that you are to present your case in a court of law, not in the court of public opinion. We respect the process, we respect the judge and I'm not going to comment."